Several techniques are known in the art for extinguishing arcs that are formed between a fixed contact and a movable contact during the switching operation of a contactor, including magnetic blowout. For magnetic blowout, a coil or a permanent magnet is arrange in the vicinity of the contacts such that the direction of the magnetic field is perpendicular to the arc. The magnetic field will the exert a Lorentz force on the arc by means of which the arc is driven in a desired direction, for example into an arc chamber, where the arc is brought into contact with splitter plates for cooling and extinguishing.
From reference DE 1 246 851, for instance, a direct current switching apparatus is known, wherein two oppositely directed permanent magnets are arranged in the vicinity of each of the two contacts of a contact bridge in order to blow-out, depending on the direction of the current flow, at least one of the two arcs.
A similar configuration is also known from US patent application US 2008/0030289. According to this reference, a contactor with a contact bridge and two oppositely directed permanent magnets, arranged in the vicinity of the two contact points, is provided. Depending on the direction of the current flow through the switch, either one of the two arcs is blown into an arc extinguishing chamber, where it will be quickly eliminated.
The problem with conventional contactors having permanent magnets for magnetic blowout, however, is that it can hardly be avoided that the blowout field is also affecting the contact bridge on which the movable contacts are mounted. Hence, the magnetic field will also exert a Lorentz force on the contact bridge. If the blowout magnets are arranged such that the arcs are pushed out of the switch, as required for arc extinguishing, this force will also tend to push the contact bridge in the very same direction. In a worst case scenario with very high currents (typically larger than 1 kA) over closed contacts, this force may overcome the contact holding force, resulting in an uncontrolled opening of the contacts. The arc generated in such a situation will rapidly produce an enormous amount of heat, leading to complete destruction of the contactor.
From U.S. Pat. No. 5,815,058 a high current switch is known that is equipped with a contact enhancement apparatus for preventing the contact from being opened by repulsive magnetic forces generated by a large current flowing through the switch in a fault situation. According to this reference, the contact bridge of the switch is provided with an iron bar in order to generate an attractive electromagnetic force to another iron bar located beneath the contact bridge. This attractive electromagnetic force will counteract the repulsive force so as to ensure that the contact remains close, even when a large current is flowing through the switch. This switch, however, is not equipped with an arc extinguishing means.